Objective Butyrophilin 1A1 (BTN1A1) gene from Symphysodon aequifasciatus was cloned and its express pattern under pathological infection was analyzed, to provide basis for better understanding of the function of BTN1A1 gene in S. aequifasciatu.

Method The structure and evolutionary characteristics of two BTN1A1 genes (BTN1A1-1 and BTN1A1-2) were analyzed by bioinformatics analysis. Based on the transcriptome data of the skin of the S. aequifasciatu obtained at early stage, the CDS regions of BTN1A1-1 and BTN1A1-2 genes were used to design the primers for gene cloning. The expressions of BTN1A1-1 gene in various tissues and its expression pattern under the stimulus of Aeromonas hydrophila were analyzed by qRT-PCR.

Result The ORF regions of BTN1A1-1 and BTN1A1-2 were 894 and 1 275 bp, encoding 298 and 424 amino acids, respectively. Both BTN1A1-1 and BTN1A1-2 had a classical structure of BTN, which contained a signal peptide, an Ig domain, an lg_like domain and a transmembrane domain. BTN1A1-2 also had a cytoplasmic domain. BTN1A1-1 showed a closer relationship with the genes found in cichlid fish. It shared the highest homology of 94.14% with the corresponding amino acid sequences of Archocentrus centrarchus; while BTN1A1-2 showed lower similarity with other types of fish, forming a special branch. The expressions of BTN1A1-1 and BTN1A1-2 were highly in gill, intestine and skin of S. aequifasciatu. The expression of BTN1A1-1 decreased significantly and then increased after the infection of A. hydrophila, while the expression of BTN1A1-2 increased significantly and then decreased.

Conclusion The BTN1A1-1 gene was more evolutionarily conservative than BTN1A1-2. The differential expressions of BTN1A1-1 and BTN1A1-2 in response to immune stimulation suggests that they may be functionally differentiated in the immune defence of S. aequifasciatus.